Reverse osmosis (RO) membranes are widely used in the separation of water from various aqueous solutions, for instance for water desalination. Such membranes have been made for example from cellulose acetate. The processes for the preparation of such membranes are described in Loeb et al. U.S. Pat. Nos. 3,133,132 and 3,133,137, in Cannon U.S. Pat. No. 3,460,683 and in MacDonald et al. U.S. Pat. No. 3,842,515. The processes involve generally: preparation of a suitable casting solution, casting the solution onto a substrate, evaporation of the solvent and gelling the cast in a gelling medium. The casting solutions used in the processes referred to hereinabove generally contain acetone.
It has also been known for a number of years to manufacture membranes by regenerating cellulose from cellulose acetate or from cellulose nitrate.
While cellulose acetate membranes are useful, cellulose membranes have a distinct advantage over the former. The advantage is in their higher mechanical strength and lower affinity to organic solute compounds in aqueous solutions. Thus, cellulose membranes are less susceptible to fouling by organic contaminants. The relatively small affinity to organic compounds also enables celulosic membranes to withstand the environment of nonaqueous solvents.
It is also known that membranes may be obtained from cellulose by dissolving a cellulose-containing material in dimethyl sulfoxide (DMSO) and paraformaldehyde (PF). These two compounds appear to be excellent and nondegrading solvents of cellulose when applied in certain proportions. The formation of methylol cellulose is reported to be crucial to the dissolution mechanism. The DMSO/PF solvent system is capable of dissolving a variety of cellulosic materials with weight average degrees of polymerization ranging from 16 to over 8000.
Seymour and Johnson, Journal of Applied Polymer Science, Vol. 20, 3425 (1976), have attempted to prepare cellulose solutions in the DMSO/PF solvent with concentrations up to 10%. The temperature of dissolution in these experiments was 65.degree. to 80.degree. C., and it was found that complete cellulose solution was only achieved with cellulose concentration about 0.5%. The molar ratio of paraformaldehyde to cellulose was about 10:1. Such mixtures of cellulose and PF in dimethyl sulfoxide produced homogeneous solutions or gels from which cellulose was precipitated by adding dioxane, alcohol or water. However, precipitation in water or ethanol, wet spinning in water or alcohol as well as solvent evaporation resulted in brittle films and weak fibers.
(a) dissolving cellulose (or cellulosic material) in a dimethyl sulfoxide/paraformaldehyde solvent to form a casting solution containing PA1 (b) cooling the casting solution to about room temperature, PA1 (c) casting the cooled solution at about room temperature, PA1 (d) evaporating the solvent from the cast solution at a temperature in the range 140.degree.-180.degree. C., and PA1 (e) gelling the cast solution in a gelling medium comprising at least one liquid selected from the group consisting of water, an alcohol having carbon atoms in the range 1 to 4, acetone and methyl ethyl ketone at a temperature of 0.degree.-30.degree. C.
(i) 12-15 wt % cellulose, PA2 (ii) 70-76 wt % dimethyl sulfoxide, and PA2 (iii) 12-15 wt % paraformaldehyde at an elevated temperature while formaldehyde gas is evolved and until a transparent casting solution is formed,
Preferably, the temperature of evaporation is about 170.degree. to 175.degree. C. The gelling medium must be a protic solvent to achieve a satisfactory gellation.
Following the work of Seymour and Johnson, Doshi and Webb reported a relation between the concentration of cellulose in the casting solution (or in the gel) and the pore sizes of the membranes obtained thereby (Proceedings of 1983 International Dissolving and Specialty Pulps Conference, Boston, April 5-8, 1983, TAPPI Press, Atlantic, Ga., 41 (1983)). The membranes obtained by Doshi and Webb had pore sizes of ca. 750.times.10.sup.-10 m (750 .ANG.).
Uragami et al. (Sep. Sci. and Tech., 17(2), 307 (1982) obtained casting solutions based on the dissolution method of Seymour and Johnson with cellulose concentrations below 9 wt. %, wherein the solvent was evaporated below 110.degree. C.
For certain reverse osmosis applications such as water desalination, it is desirable to obtain membranes of pore radii smaller than 30.times.10.sup.-10 m. Such membranes can be produced from cellulose acetate but have not thus far been obtained from cellulose. Such need exists in view of the advantages of cellulosic membranes as explained above.